In recent years, speed of electric signals (frequencies of electric signals) to be transmitted to a control printed circuit board, which is incorporated in an electric device such as a car navigation system of an automobile and on which components such as an electronic component and an IC (an integrated circuit) are mounted, has been increased. In addition, a circuit pattern on such a printed circuit board has become denser. Generally, a shielded cable is used to transmit high frequency electric signals, and along with the increase in frequencies of electric signals, a high frequency shielded connector to be connected to the shielded cable has been increasingly required.
Examples of the shielded cable include a so-called coaxial cable. A coaxial cable generally has a coaxial structure in which a signal conductor which is used as a transmission path of electric signals and is defined by a bundle of a plurality of elemental wires such as copper wires, a shielded conductor which is defined by a braided wire composed of a plurality of elemental wires, an insulator which is interposed between the conductors, and an insulating sheath which is arranged to cover the shielded conductor are concentrically arranged. The shielded conductor covers the signal conductor leaving no clearance and electromagnetically shields the signal conductor.
Generally, a shielded connector to be connected to an end of the coaxial cable which transmits high frequency electric signals is provided with an inner conductor terminal to be connected to the signal conductor which transmits high frequency electric signals, an outer conductor terminal to be connected to the shielded conductor defined by the braided wire and arranged to cover the inner conductor terminal, and a dielectric having a predetermined dielectric constant which is provided between the inner conductor terminal and the outer conductor terminal. The insulator and the sheath at the end of the coaxial cable are stripped off to expose the signal conductor and the shielded conductor, and the inner conductor terminal and the outer conductor terminal are electronically connected to the exposed signal conductor and the exposed shielded conductor respectively.
An example of a conventional shielded connector is disclosed in Japanese Patent Application Unexamined Publication No. 2000-173725. FIG. 9 is a longitudinal sectional view of the shielded connector. As shown in FIG. 9, a shielded connector 20 is connected to portions of a signal conductor Wa and a shielded conductor Wd of a coaxial cable W which are exposed by stripping off an insulator Wb and a sheath We. An inner conductor terminal 21 is connected to the signal conductor Wa via a crimp section 21a, and an outer conductor terminal 23 is connected to the shielded conductor Wd via a crimp section 23a. A dielectric 22 which brings the terminals into an insulated state is interposed between the terminals.
Generally, a characteristic impedance of a coaxial cable in the transmission of high frequency electric signals is set to be, for example, 50 ohms in order to match with a characteristic impedance of a printed circuit board of an electric device to which the coaxial cable is connected. If there is a portion such that the characteristic impedances do not match with each other (a mismatch portion) in a transmission path of high frequency electric signals, reflection of electric signals occurs in the mismatch portion, thereby causing problems such as reduction in transmission efficiency and generation of noise. Therefore, it is necessary to match the characteristic impedance of the shielded connector to the characteristic impedance of the coaxial cable.
In general, impedance matching of the characteristic impedance of the shielded connector to the characteristic impedance of the coaxial cable is obtained by adjusting “the ratio of an inside diameter of a shell portion of an outer conductor terminal to an outer diameter of a terminal section of an inner conductor terminal” and “a dielectric constant of a dielectric”. As shown in FIG. 9, an outer diameter of the crimp section 21a of the inner conductor terminal 21 after being crimped is made to have a size and shape giving a higher priority to reliability of electrical connection with the signal conductor Wa, and is generally smaller than an outer diameter of a terminal section 21b. Thus, “the ratio of an inside diameter of a shell portion 23b of the outer conductor terminal 23 to an outer diameter of the crimp section 21a of the inner conductor terminal 21” at the crimp section 21a is not equal to “the ratio of the inside diameter of the shell portion 23b of the outer conductor terminal 23 to the outer diameter of the terminal section 21b of the inner conductor terminal 21” at the terminal section 21b. Therefore, the characteristic impedance in the crimp section 21a of the inner conductor terminal 21 does not match with the characteristic impedance of the coaxial cable W and becomes higher than it.
In a section in which the characteristic impedance of the shielded connector is not equal to the characteristic impedance of the coaxial cable, reflection or radiation of transmitted electric signals occurs, which brings about such problems that electric signals are not properly transmitted and that noise is generated. Especially in the transmission of high frequency electric signals of several gigahertzes, the problems remarkably occur.
In order to remedy the problems, the characteristic impedance at the crimp section of the inner conductor terminal is to be lowered and matched to the characteristic impedance of the coaxial cable, and impedance matching can be obtained by enlarging the outer diameter of the crimp section of the inner conductor terminal after being crimped to be the same as the outer diameter of the terminal section of the inner conductor terminal. Conventionally, a method of attaching a cylindrical metal sleeve to the crimp section is used for enlarging the outer diameter of the crimp section.